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      mongodb笔记(十) 分片
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			<h2 class="toc-title"><span>Contents</span></h2>
		
			<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#分片简介"><span class="toc-number">1.</span> <span class="toc-text">分片简介</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#MongoDB中的自动分片"><span class="toc-number">2.</span> <span class="toc-text">MongoDB中的自动分片</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#片键"><span class="toc-number">3.</span> <span class="toc-text">片键</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#建立分片"><span class="toc-number">4.</span> <span class="toc-text">建立分片</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#启动服务器"><span class="toc-number">4.1.</span> <span class="toc-text">启动服务器</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#切分数据"><span class="toc-number">4.2.</span> <span class="toc-text">切分数据</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#生产配置"><span class="toc-number">5.</span> <span class="toc-text">生产配置</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#健壮的配置"><span class="toc-number">5.1.</span> <span class="toc-text">健壮的配置</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#多个mongos"><span class="toc-number">5.2.</span> <span class="toc-text">多个mongos</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#健壮的片"><span class="toc-number">5.3.</span> <span class="toc-text">健壮的片</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#物理服务器"><span class="toc-number">5.4.</span> <span class="toc-text">物理服务器</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#管理分片"><span class="toc-number">6.</span> <span class="toc-text">管理分片</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#配置集合"><span class="toc-number">6.1.</span> <span class="toc-text">配置集合</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#分片命令"><span class="toc-number">6.2.</span> <span class="toc-text">分片命令</span></a></li></ol></li></ol>
		
		</div>
		
        <h2 id="分片简介">分片简介</h2><p>分片是指将数据拆分，将其分散存在不同的机器上的过程。</p>
<a id="more"></a>
<h2 id="MongoDB中的自动分片">MongoDB中的自动分片</h2><p>没有分片的时候，客户端连接mongod。分片时客户端会连接到mongos，mongos对应用隐藏了分片的细节。</p>
<p>何时分片 </p>
<ol>
<li>机器的磁盘不够用了</li>
<li>单个mongod已经不能满足写数据的性能需要了。</li>
<li>想将大量的数据放在内存中提高性能。</li>
</ol>
<p>一般来说，先从不分片开始，然后在需要时将其转换成分片。</p>
<h2 id="片键">片键</h2><p>在设置分片时，需要从集合里面选一个键，用该键的值作为数据拆分的依据。这个键就是片键。</p>
<p>例如我有个文档集合表示人员，如果选择名字(name)为片键，第一片可存放A~F开头的文档，第二片存放G~P的文档，第三片存放Q~Z的文档。</p>
<p>选择片键并创建片键很像索引，因为二者原理类似。事实上，片键也是最常用的索引。</p>
<p>片键对操作的影响<br><figure class="highlight openscad"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">db.people.find<span class="params">(<span class="params">(&#123;<span class="string">"name"</span>:<span class="string">"Susan"</span>&#125;)</span>)</span>   <span class="comment">//mongos会将这个查询直接发送给Q~Z片</span></span><br><span class="line">db.people.find<span class="params">(&#123;<span class="string">"name"</span>:&#123;<span class="string">"$lt"</span>:<span class="string">"L"</span>&#125;&#125;)</span>    <span class="comment">//mongos会将这个查询先后发送给A~F和G~P片</span></span><br><span class="line">db.people.find<span class="params">(&#123;<span class="string">"email"</span>:<span class="string">"joe@example.com"</span>&#125;)</span>   <span class="comment">//mongos不跟踪email键，所以不知道应该讲查询发给谁，就向所有片发送查询</span></span><br></pre></td></tr></table></figure></p>
<h2 id="建立分片">建立分片</h2><p>建立分片只有两步：启动实际的服务器，然后决定怎么切分数据。<br>分片一般有3个组成部分：</p>
<ul>
<li><p>片</p>
<p>  就是保存子集合数据的容器。片可以是单个mongod服务器，也可以是副本集。所以即使一片内有多台服务器，也只能有一个主服务器。</p>
</li>
<li><p>Mongos</p>
<p>  是MongoDB各版本中配的路由器进程。它路由所有请求，然后将结果聚合。它本身并不存储数据或者配置信息（但会缓存配置服务器的信息）。</p>
</li>
<li><p>配置服务器</p>
<p>  存储了集群的配置信息，数据和分片的对应关系。Mongos不永久存放数据，所以需要个地方存放分片配置，它会从配置服务器获取同步数据。</p>
</li>
</ul>
<h3 id="启动服务器">启动服务器</h3><p>首先启动配置服务器和mongos。配置服务器需要先启动，因为mongos会用到配置信息。配置服务器的启动就像普通的mongod一样。<br><figure class="highlight brainfuck"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">mkdir</span> <span class="literal">-</span><span class="comment">p</span> <span class="comment">~/dbs/config</span></span><br><span class="line"><span class="string">.</span><span class="comment">/mongod</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">dbpath</span> <span class="comment">~/dbs/config</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">port</span> <span class="comment">20000</span></span><br></pre></td></tr></table></figure></p>
<p>建立mongos进程，这种路由服务器连接数据目录都不需要，但一定要指明配置服务器的位置。<br><figure class="highlight brainfuck"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="string">.</span><span class="comment">/mongos</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">port</span> <span class="comment">30000</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">configdb</span> <span class="comment">localhost:20000</span></span><br></pre></td></tr></table></figure></p>
<p>添加片</p>
<p>片就是普通的mongod实例（或者副本集）<br><figure class="highlight brainfuck"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">mkdir</span> <span class="literal">-</span><span class="comment">p</span> <span class="comment">~/dbs/shard1</span></span><br><span class="line"><span class="string">.</span><span class="comment">/mongod</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">dbpath</span> <span class="comment">~/dbs/shard1</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">port</span> <span class="comment">10000</span></span><br></pre></td></tr></table></figure></p>
<p>现在连接刚才启动的mongos，为集群添加一个片。启动shell，连接mongos<br><figure class="highlight aspectj"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">./mongo localhost:30000/admin</span><br><span class="line">db.runCommand(&#123;</span><br><span class="line"><span class="string">"addshard"</span> : <span class="string">"localhost:10000"</span>,</span><br><span class="line"><span class="string">"allowLocal"</span> : <span class="keyword">true</span></span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure></p>
<p>当在localhost上运行片时，得设定allowLocal键。</p>
<h3 id="切分数据">切分数据</h3><p>MongoDB不会将存储的每一条数据都直接发布，得先在数据库和集合的级别将分片功能打开。下面例子我们以”_id”为基准切分foo数据库的bar集合。<br>首先开启数据库foo的分片功能：<br><figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">db.<span class="function"><span class="title">runCommand</span><span class="params">(&#123;<span class="string">"enablesharding"</span>:<span class="string">"foo"</span>&#125;)</span></span></span><br></pre></td></tr></table></figure></p>
<p>然后开启集合bar的分片功能并以_id为片键<br><figure class="highlight openscad"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">db.runCommand<span class="params">(&#123;</span><br><span class="line"><span class="string">"shardcollection"</span>:<span class="string">"foo.bar"</span>,</span><br><span class="line"><span class="string">"key"</span>:&#123;<span class="string">"_id"</span>:<span class="number">1</span>&#125;</span><br><span class="line">&#125;)</span></span><br></pre></td></tr></table></figure></p>
<h2 id="生产配置">生产配置</h2><p>成功地构建分片需要如下条件：</p>
<ul>
<li>多个配置服务器</li>
<li>多个mongos服务器</li>
<li>每个片都是副本集</li>
<li>正确设置w (有关w和复制的信息在上一章)</li>
</ul>
<h3 id="健壮的配置">健壮的配置</h3><figure class="highlight brainfuck"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">mkdir</span> <span class="literal">-</span><span class="comment">p</span> <span class="comment">~/dbs/config1</span> <span class="comment">~/dbs/config2</span> <span class="comment">~/dbs/config3</span></span><br><span class="line"><span class="string">.</span><span class="comment">/mongod</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">dbpath</span> <span class="comment">~/dbs/config1</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">port</span> <span class="comment">20001</span></span><br><span class="line"><span class="string">.</span><span class="comment">/mongod</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">dbpath</span> <span class="comment">~/dbs/config2</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">port</span> <span class="comment">20002</span></span><br><span class="line"><span class="string">.</span><span class="comment">/mongod</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">dbpath</span> <span class="comment">~/dbs/config3</span> <span class="literal">-</span><span class="literal">-</span><span class="comment">port</span> <span class="comment">20003</span></span><br></pre></td></tr></table></figure>
<p>创建配置服务器如上，现在启动mongos的时候应将其连接到这3个配置服务器。<br><figure class="highlight groovy"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">./mongos --configdb <span class="string">localhost:</span><span class="number">20001</span>,<span class="string">localhost:</span><span class="number">20002</span>,<span class="string">localhost:</span><span class="number">20003</span></span><br></pre></td></tr></table></figure></p>
<p>配置服务器使用的是两步提交机制，不是普通的MongoDB的异步复制，来维护集群配置的不同副本。这样能保证集群状态的一致性。这意味着某台配置服务器宕掉了后，集群配置信息将是只读的。但是客户端还是能够读写的，只有所有配置服务器备份了以后才能重新均衡数据。</p>
<h3 id="多个mongos">多个mongos</h3><p>mongos的数量不受限制，建议针对一个应用服务器只运行一个mongos进程。这样每个应用服务器就可以与mongos进行本地会话。</p>
<h3 id="健壮的片">健壮的片</h3><p>生产环境中，每个片都应是副本集。这样单个服务器坏了，就不会导致整个片失效。用addshard命令就可以讲副本集作为片添加，添加时只要指定副本集的名字和种子就好了。</p>
<p>比如现在添加副本集foo，其中包含一个服务器prod.example.com:27017 (还有别的服务器)，就可以使用下列命令将其添加到集群里。<br><figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">db.<span class="function"><span class="title">runCommand</span><span class="params">(&#123;<span class="string">"addshard"</span>:<span class="string">"foo/prod.example.com:27017"</span>&#125;)</span></span></span><br></pre></td></tr></table></figure></p>
<p>如果prod.example.com挂了，则mongos会知道它所连接的是哪个副本集，并去寻找新的主节点。</p>
<h3 id="物理服务器">物理服务器</h3><p>不把所有配置服务器放到一台机器，不把所有mongos放到一台机器，不把副本集放到一台机器，但是可以把一个配置服务器，一些mongos进程和副本集的一个节点放到一台服务器上。</p>
<h2 id="管理分片">管理分片</h2><p>分片的信息主要存放在config数据库上，这样就能被任何连接到mongos的进程访问到了。</p>
<h3 id="配置集合">配置集合</h3><p>下面的代码都假设已经在shell中连接了mongos，并且已经运行了use config。</p>
<ol>
<li><p>片</p>
<p> 可以在shards集合中查到所有的片<code>db.shards.find()</code></p>
</li>
<li><p>数据库</p>
<p> databases集合含有已经在片上的数据库列表和一些相关信息。</p>
<figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">db<span class="class">.databases</span><span class="class">.find</span>()</span><br></pre></td></tr></table></figure>
<ul>
<li>“_id” ： 表示数据名</li>
<li>“partitioned” ： 是否启用分片功能</li>
<li>“primary” ：这个值与”_id”对应，表示这个数据的大本营在哪里。也就是开始创建数据库文件的位置</li>
</ul>
</li>
<li><p>块</p>
<p> 块信息保存在chunks集合中，你可以看到数据到底是怎么切分到集群的。</p>
<figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">db<span class="class">.chunks</span><span class="class">.find</span>()</span><br></pre></td></tr></table></figure>
</li>
</ol>
<h3 id="分片命令">分片命令</h3><ol>
<li><p>获得概要</p>
<p> db.printShardingStatus() 给出前面说的那些集合的概要</p>
</li>
<li><p>删除片</p>
<p> 用removeshard就能从集群中删除片，removeshard会把给定片上的所有块都挪到其他片上。</p>
<figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">db.<span class="function"><span class="title">runCommand</span><span class="params">(&#123;<span class="string">"removeshard"</span>:<span class="string">"localhost:10000"</span>&#125;)</span></span></span><br></pre></td></tr></table></figure>
</li>
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Updated:<time datetime="2014-12-24T07:50:09.000Z" itemprop="dateModified">2014-12-24</time>
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        <ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#分片简介"><span class="toc-number">1.</span> <span class="toc-text">分片简介</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#MongoDB中的自动分片"><span class="toc-number">2.</span> <span class="toc-text">MongoDB中的自动分片</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#片键"><span class="toc-number">3.</span> <span class="toc-text">片键</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#建立分片"><span class="toc-number">4.</span> <span class="toc-text">建立分片</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#启动服务器"><span class="toc-number">4.1.</span> <span class="toc-text">启动服务器</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#切分数据"><span class="toc-number">4.2.</span> <span class="toc-text">切分数据</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#生产配置"><span class="toc-number">5.</span> <span class="toc-text">生产配置</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#健壮的配置"><span class="toc-number">5.1.</span> <span class="toc-text">健壮的配置</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#多个mongos"><span class="toc-number">5.2.</span> <span class="toc-text">多个mongos</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#健壮的片"><span class="toc-number">5.3.</span> <span class="toc-text">健壮的片</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#物理服务器"><span class="toc-number">5.4.</span> <span class="toc-text">物理服务器</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#管理分片"><span class="toc-number">6.</span> <span class="toc-text">管理分片</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#配置集合"><span class="toc-number">6.1.</span> <span class="toc-text">配置集合</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#分片命令"><span class="toc-number">6.2.</span> <span class="toc-text">分片命令</span></a></li></ol></li></ol>
    
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